Since silicon carbide (hereinafter referred to as SiC) is superior in heat resistance and mechanical strength, and is physically and chemically stable, it is attracting attention as an environment resistant semiconductor material. In recent years, there is also an increasing demand for epitaxial SiC single crystal wafers as substrates for high-frequency, high-voltage electronic devices and the like.
When a power device, a high-frequency device or the like are fabricated using a SiC single crystal substrate (hereinafter referred to as a SiC substrate), a method called thermal CVD (thermal chemical vapor deposition) is generally used to effect the epitaxial growth of a SiC thin film or to inject a dopant directly by ion injection method. In the latter case, however, annealing at high temperature is required after injection, and therefore thin film formation by epitaxial growth is frequently used.
On the epitaxial film of SiC, epitaxial defects such as triangular defects, carrot defects, and comet defects may be present and devices having these defects have markedly deteriorated characteristics. Therefore, they are known as so-called “device killer defects”. Thus, techniques for reducing epitaxial defects have been developed, one of which is a technique of fabricating an epitaxial film in a two-layer structure comprising a buffer layer and a drift layer. By this technique, in most cases, epitaxial defects are reduced in such a way that a drift layer is made to have a thickness or doping density required for device fabrication, and, in order to form a layer as a buffer layer, a layer having a doping density intermediate between the SiC substrate and the drift layer is grown, and thereby the difference between the doping densities of both is relaxed in order to reduce distortion, which leads to reduced epitaxial defects.
Patent Document 1 proposes to control the dopant concentration in a gradually decreasing manner during the growth process of the SiC epitaxial film in order to suppress the propagation of the basal plane dislocation from the SiC single crystal wafer to the SiC epitaxial film. Further, Patent Document 1 proposes to suppress conversion from the misfit dislocation to the threading edge dislocation by controlling the C/Si molar ratio in the source gas to about 0.3 to 3 and controlling the growth rate of the SiC epitaxial film to 5 μm/h or more. It is known that threading edge dislocation can become a lifetime killer of minor carriers, and since it deteriorates the device characteristics, according to the invention disclosed in Patent Document 1, the yield of the fabricated device can be improved.
Although such technologies have reduced device killer defects and dislocations that deteriorate device characteristics as described above, it has been found, in recent years, that minute pits (shallow pits) on the epitaxial film adversely affect the device characteristics (See Non-Patent Document 1). Non-patent document 1 has shown that shallow pits particularly increase the reverse leakage current of the Schottky barrier diode. The increase in the reverse leakage current is thought to be caused by the concentration of the electric field at the pit portion. Therefore, in order to improve the device characteristics and yield, it is necessary to reduce not only device killer defects but also these shallow pits.
Shallow pits have a substantially triangular shape and generally have a depth of about 50 to 80 nm, and are contained in the SiC epitaxial film at a density of 500 to 1000 pieces/cm2. The shape and depth of shallow pits are believed to be related to, for example, the pretreatment of the SiC substrate performed before growth, the ratio of the number of carbon to silicon (C/Si ratio) contained in the material gas during growth, the growth rate, the growth temperature and the like. However, studies so far have also suggested the combined effects of them.
As described below, since shallow pits are formed by virtue of the screw dislocation of the SiC single crystal wafer, the shape and depth of the shallow pits also depend on the quality of the SiC substrate itself. Since the screw dislocation of the SiC single crystal wafer generally differs greatly from substrate to substrate, it is currently difficult to stably reduce shallow pits. In the case of the production method relating to the reduction of penetration edge dislocations disclosed in Patent Document 1, it is considered difficult to reduce the shallow pits associated with the screw dislocation of the SiC substrate.
Patent Document 2 discloses a method for reducing the device killer defects described above. According to this method, at least one inhibition layer having a surface roughness Ra value of 0.5 nm or more and 1.0 nm or less is epitaxially grown at a C/Si ratio of 1.0 or less, and then an active layer of a silicon carbide single crystal thin film is epitaxially grown on the above inhibition layer at a C/Si ratio of 1.0 or more. Patent Document 2 reports that by controlling the C/Si ratio to 1.0 or less at the initial stage of epitaxial growth, it becomes to be increasingly likely that the occurrence of spiral growth originating from screw dislocation is suppressed and the dislocation is covered with a large amount of surrounding step-flows. However, as described below, the inventors of the present invention have found that, since the thickness of the inhibition layer is 1 μm or less, it is difficult with such a level of thickness to sufficiently cover the spiral step around the shallow pits with steps in the step-flow direction.
In addition, Patent Document 3 discloses a silicon carbide semiconductor device, in which a Schottky junction was provided on the silicon carbide semiconductor by depositing a high melting point metal on the surface of a SiC epitaxial layer to form a layer which is made of alloy of the high melting point metal and the SiC epitaxial layer. However, Patent Document 3 does not disclose any reduction of device killer defects from the SiC epitaxial layer.
Accordingly, although the epitaxial SiC single crystal wafer is expected to be applied to devices in the future, it will be difficult to fabricate electronic devices having excellent characteristics with high yield unless an epitaxial film with reduced influence from shallow pits can grow without depending on the quality of the SiC substrate.